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Bacteria infecting a macrophage. Coloured scanning electron micrograph (SEM) of Mycobacterium tuberculosis bacteria (purple) infecting a macrophage white blood cell. The macrophage, when activated, will engulf (phagocytose) the bacteria and destroy them as part of the body's immune response. M. tuberculosis causes the disease tuberculosis. It infects the lungs creating primary tubercles, nodular lesions of dead tissue and bacteria. The bacteria may enter the blood or lymph systems and spread to other organs of the body. Tuberculosis is spread by coughs and sneezes.